It is customary to configure electrical resistance heaters with a bare electrical conductor in filament form spirally wound about the central axis of a cylindrical cavity in the heater housing and to provide water inlet and outlet ports of the housing in communication with the cylindrical cavity.
At low rates of flow, e.g., three-tenths to four-tenths of a gallon per minute, the flow through the tube is lamellar, such that there is little cross-mixing. As a consequence, the water flowing in close proximity to the filament can become overheated and cause cavitation, wherein air collects in a large bubble and the filament burns out. Thus, the burned out portion experiences heating in air at a Watts density that is only suitable for totally submerged operation. Such events are manifestly operational shortcomings of known heaters which lessen their effectiveness.